A final word of caution may be given. Economic rent is not confined to the cases of contract rent. It exists in every case where a more or less durable agent yields a use that is scarce and desirable. The owner who uses a thing himself gets the advantage in the product as clearly as if he collected rent from a borrower. Houses lived in by the owners, house furnishings, clothing, books, all scarce and durable agents, are yielding rents in this logical sense. To the economist, therefore, the problem of economic rent, as one of the grand divisions of the problem of value, remains of undiminished importance, for in these unceasing streams of uses emanating from our environment, is found the basis for the value of all durable wealth. CHAPTER 9 THE LAW OF DIMINISHING RETURNS § I. DEFINITION OF THE CONCEPT OF (ECONOMIC) DIMINISHING RETURNS [Sidenote: Economic agents contain uses to be obtained only with progressive difficulty] 1. _The phrase "diminishing returns of industrial agents" is the expression of the fact that there is an elastic limit to the utility any indirect good can afford within a given time._ Successive attempts to get additional services from a thing are usually in part successful, but each additional service is gained with more difficulty, or a smaller added service is gained for an equal expenditure of materials or effort. A book stands many hours untouched on the shelves of the library; but if, as often happens, two or more persons wish to use it at the same hour, time and energy are wasted. The book has a potential use during the twenty-four hours, but all this can be secured only at the cost of the greatest inconvenience. The greatest net uses, therefore, are seen to be to the first user and in the first hour, for these uses cost the least time and trouble. If the members of a family will take turns, one chair will serve for all of them; but if all are to be able to sit down together, a chair must be provided for each. Often it will happen that only one chair is in use, the other nine chairs being valued only for their potential uses. I knew two young men who owned a dress-coat in partnership, and as they had different evenings free from business all went well until both were invited to a reception which both were very eager to attend. [Sidenote: This is true of all classes of agents] Illustrations of this principle may be drawn from every class of durable goods. The example generally given is that of a field used for agriculture. It was long ago seen that a larger crop could usually be obtained on the same area, only with greater effort or expenditure; but this fact has been thought to be peculiar to the use of land. The examples given above have been purposely chosen from very different fields, to show that the truth is a general one: a good that affords a given service can be made to increase that service, ordinarily, only on condition that men put forth greater effort, or sacrifice more goods.